I really hate that Sasuke is winning against Itachi, but I don't find it that unbelievable. Sasuke has two main things going for him. First, the reason everyone else is afraid of Itachi is his genjutsu and Sasuke is the first character we've seen that has similar level abilities in that aspect. Secondly, Sasuke has spent almost every day of his life planning out this fight. He was able to break out of Itachi's Tsukuyomi because he had experienced before and had analyzed it.